Why solids expand on heating

Question

I came upon the following question:

A solid expands on heating because:

a) the potential energy of interaction between atoms in the solid is asymmetric about the equilibrium positions of atoms; or

b) the frequency of vibrations of atoms increases.

My attempt:

I did not make a huge deal out of the question, and simply thought that it is due to the increase in frequency of vibrations, which now causes greater oscillations about the lattice points, which in turn increases the average volume occupied by atoms, leading to expansion. However the answer is a).

Can someone explain, possibly using some model (like the spring-particle model of the lattice), why this is so, and why b) is NOT a cause of expansion?

Answer 1

A simple and naïve model* for thermal expansion has two neighboring atoms represented by a pair of balls connected by a spring.

^{Image source: General Physics (calculus based) Class Notes, Dr. Rakesh Kapoor from University of Alabama at Birmingham}

Giving the balls more energy (increasing the temperature of the atoms) will increase the amplitude of the spring's oscillations, which increases the amount of space the atoms will take up. The frequency of oscillations—how quickly the balls move back and forth—is irrelevant.

As for what "asymmetry of atom equilibrium positions" has to do with it, take a look at the graph below. The equilibrium distance for these atoms (spring length in the ball model) is marked by the dotted line at 74 pm. Clearly the green potential energy line is not symmetric across this line. From the green line shape it can be seen that atoms at equilibrium distance require less energy input to pull apart than to compress by the same amount, explaining why the addition of energy typically results in an increase in distance rather than a decrease.

^{Image source: Chemistry 301, University of Texas (online resource)}

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*From Thermal Expansion of Solids by C. Y. Ho and R. E. Taylor:

A popular explanation for thermal expansion assumes central forces between pairs of interacting atoms. The asymmetry of the potential energy well causes the mean distance between the atoms to increase when they vibrate along the line joining them. In many texts this is the only model given for thermal expansion... although crude and in some ways misleading for solids (it does not explain negative expansion), it reveals the most generally important mechanism for thermal expansion, and correctly suggests that atomic vibrations give rise to thermal expansion because of anharmonicity.